Some radiation sources exhibit wavelength drift over time in excess of that tolerable for many applications. This drift becomes increasingly important as the lifetimes over which these radiation sources are to be deployed increases. For some applications, e.g., those having multiple channels, wavelength stability is required to be within a few percentage of the channel spacing. Factors such as temperature, age, operating power level, etc., all affect the output wavelength. Parameters such as the direction of the wavelength change, the degree of the change, and the percentage of the light being radiated at the different wavelengths may be monitored. By monitoring any or all these parameters, the radiation source may be controlled in accordance with known techniques to stabilize the output of the radiation source.
Such monitoring and stabilizing systems typically involve using a unit that is external to the radiation source itself. Such external units include crystal gratings, fiber gratings, spectrometers, and Fabry-Perot etalons, both straight and inclined. The grating systems include relatively large control units external to the radiation source. While etalon-based systems offer a more compact solution, so far these etalons are still separate units that may become improperly aligned, either with photodetectors or with optical elements required to direct and control the light onto the photodetectors. Further, etalon performance is very sensitive to angular alignment. Etalon perfomrance, particularly for solid etalons, is also very sensitive to temperature shifts.
One problem encountered in ensuring coverage of a particular wavelength region of interest is that the wavelength dependence of the output of the above configurations needs to be strong. In other words, the slope of a curve of an output versus wavelength must sufficiently steep to resolve a wavelength being detected. This leads to a requirement for multiple filters, increasing the cost and complexity of the system.
Another problem when using an etalon, or any wavelength-dependent component for which optical path length affects the output, is that these configurations typically require a collimated beam. The detectors are typically placed adjacent to these wavelength dependent components. Providing a collimated laser beam onto the detectors is inefficient, given the size limits placed on the detectors. This also leads to an increased number of wavelength dependent components needed to accurately monitor the wavelength.
Further, etalon response is periodic with respect to wavelength, so while an etalon may provide a large range of wavelength coverage, the slope in a given region may not be sufficient or may not be unique, rendering a wavelength unresolvable. This is especially problematic when dealing with fixed wavelength or narrowly tunable radiation sources and a plurality of channels, each occupying a narrow wavelength region, e.g., 50 GHz.